Ferritic stainless steel

ABSTRACT

Ferritic, preferably free-machining stainless steel containing 0.08% max. carbon, 0.06-0.20% nitrogen, 2.5% max. manganese, 0.50% max. sulfur (and/or selenium), 17.5-19.5% chromium, 1.52.5% molybdenum with the balance iron plus optional elements and incidental impurities, characterized by a unique combination of corrosion resistance and impact toughness, plus free machinability in its preferred form.

United States Patent Schlosser et al. [4 Dec. 23, 1975 1 FERRI'IICSTAINLESS STEEL 2.848.323 8/1958 Harris 75/126 1 4 2,905,577 9/1959Harris 75/l26 C [75] Invemm' M shnmgmn 3.645.722 2/1912 7411161115361115/128 P Wis R" Cam", 5.799.765 5/1914 Clarke 75/126 L both of g. all OfB4l6,399 1/1975 061mm. Jr. 15/126 L 1223.685 7/l953 Clarke [73) W TM1127.226 5/1965 Moskowitz 75/128 F Reading. Pa.

[22] Filed: 1973 Primary Examiner-R. Dean 211 App], 414,157 AssistantExaminer-Arthur J. Steiner Attorney, Agent, or Firm-Edgar N. Jay

[52] US. Cl. 148/37; 75/126 C; 75/126 J;

15/126 1.; 75Il26 M; 75/128 P; 75/128 19; 1 1 ABSTRACT 75ll28 w [51]1111. C1. c22c 38/22; c22c 38/60 mf'g'gggfg ffi $3 222 I ml $1 3;, 2.5%max. manganese, 0.50% max. sulfur (and/or selenium), l7.5l9.5% chromium,l.52.5% molybdenum with the balance iron plus optional elements and [56]Cited incidental impurities, characterized by a unique com- UNrrEDSTATES PATENTS bination of corrosion resistance and impact toughness,1.956.645 5/l934 Langenberg 75/128 P plus free machinability in itspreferred form. 2,384,565 9/[945 Schaufus 75/128 F 2.624.669 U195! 12Claims, No Drawings Crafts IS/I26 J FERRITIC STAINLESS STEEL BACKGROUNDOF THE INVENTION This invention relates to ferritic stainless steel and,more particularly, to a ferritic stainless steel characterized by aunique combination of free machinability, corrosion resistance andtoughness, the latter as measured by the V-notch Charpy impact test.

Chromium steels modified with molybdenum and sulfur have hitherto beenknown. One such steel, here designated Alloy A for convenience, aspublished, contained, nominally, 18% chromium, 2% molybdenum and 0.20%sulfur. The remainder of Alloy A was 0.15% carbon, 0.30% silicon, 0.80%manganese, a maximum of 0.040% phosphorus, and the balance iron plusincidental impurities. Another such composition, Alloy B, contained amaximum of 0.03% carbon, 17.50l9.50% chromium, a maximum of 1.50%manganese, a maximum of 1.0% silicon, a maximum of 0.04% phosphorus,0.30-0.35% sulfur, 1.50-2.50% molybdenum, and the balance iron plusincidental impurities. Depending upon how such steels were produced theymight contain up to about 0.040% nitrogen as a residual or incidentalimpurity even though none would be intentionally added. Alloys A and Bare essentially the same and are commonly designated as 18 Cr-2 Mostainless steel with additives for machinability.

We have found that such 18% Cr-2% Mo stainless steels leave much to bedesired when, in particular, room temperature toughness or impactstrength as measured by the V-notch Cbarpy impact test and where lowerimpact transition temperature are wanted in such parts as constructionfasteners for the building trade, valves for handling chemicals, machineshafts, food-processing equipment, and generally any screw machineparts, especially where any such parts would be exposed in service tosubfreezing temperatures.

SUMMARY OF THE lNVENTlON It is, therefore, a principal object of thisinvention to provide an 18% Cr-2% Mo stainless steel which has improvedtoughness and good corrosion resistance, and in its preferred form alsohas good machinability.

1n accordance with our invention a ferritic stainless steel is providedwhich in its annealed condition contains about 5-5() percent, preferablyabout 5 to 20 percent, highly tempered not fully decomposed martensite,from which such parts construction fasteners, valves for handlingchemicals, machine shafts, food-processing equipment, screw machineparts and others can be made having good corrosion resistance andtoughness and which preferably also have good machinability. By way ofsummary our composition in its broad and preferred ranges contains inweight percent and the balance iron plus incidental impurities.

DESCRIPTION OF PREFERRED EMBODIMENTS By tabulating the ranges of ourcomposition to provide a convenient summary, it is not intended torestrict our alloy to the stated broad and preferred combinationsthereof, it being intended to include within the scope of this inventionas defined by the claims, elements equivalent to those stated andcombinations of one or more of the broad ranges with one or more of thepreferred ranges.

As incidental impurities several hundredths percent of one or more ofsuch elements as phosphorus, copper and nickel may be included, but, nomore than 0.04% phosphorus, 0.5% copper or 0.5% nickel. Strongstabilizing elements such as titanium and columbium are not desirableadditions to our alloy because they tie up nitrogen. Selenium can besubstituted for all or part of the sulfur on a 1 for 1 basis, andtungsten can be substituted for all or part of the molybdenum in theratio of about 1.5 to 1.

Silicon is not an essential alloying addition to our alloy, but it ispreferably used in the customary way for deoxidatiou. With less than0.2% silicon, deoxidation is not usually carried far enough, and aboveabout 0.6%, the silicon has the objectionable effect of formingundesired silicates and tends to raise the impact transition temperatureof the composition. Thus, while up to about 1% silicon is tolerable,02-06 percent best favors desired deoxidation and, by its effect onmicrostructure, optimum mechanical properties.

Both nickel and copper are not desired in our composition. Though eachis tolerable in amounts up to about 0.5 percent, neither should bepresent in excess of that amount because of their adverse effect uponthe resistance to cracking while under stress when exposed to such mediaas hot chlorides.

Carbon, in addition to being a strong austenite former, results in theformation of undesired grain boundary constituents and is, therefore,limited to no more than about 0.08 percent, preferably no more thanabout 0.06 percent. When carbon is present in an amount of no less thanabout 0.02 percent, the minimum amount of nitrogen required inaccordance with the present invention need not be adjusted. However, ifthe carbon content is reduced below about 0.01 percent, the minimum ofnitrogen required should be adjusted upward from 0.06 percent by anequal amount.

At least about 0.06% nitrogen is required in our composition andpreferably at least 0.08 percent to provide the toughness and reducedimpact transition temperature of the composition. By impact transitiontemperature is meant the lowest temperature at which the impactspecimens show predominantly ductile fracture. Up to about 0.20%nitrogen can be included to offset the maximum permissible amounts ofthe ferrite-forming elements which include chromium and molybdenum andthereby ensure the required minimum amount of martensite. Preferably0.08-0.16% or, better yet, 0.08-0. 14% nitrogen is present with thelarger amounts of nitrogen being balanced with the larger amounts ofchromium and molybdenum so that at least the small but definite amountof martensite required, about 5-20 percent, is present when, followinghot working, the material is to be annealed. However, it is to be notedthat when the maximum amounts of carbon and nitrogen in the broad rangeare used together as much as about may be martensite following hotworking.

Manganese and sulfur (and/or selenium) work together and are added toprovide the best free machinability in our composition. To this end, aminimum of about l.5 percent, preferably at least L60 percent, isincluded to provide with the sulfur the best freemachining propertiescharacteristic of our alloy. Manganese in excess of about 2.5 percentdoes not contribute significantly to free machinability. Preferably,manganese is limited to no more than about 2.2 percent, and best resultsare provided with about l.72.2% manganese. Below about 0.15 percent,sulfur is not present in sufficient quantity to prevent the formation oflong stringy chips which tend to clog the machine and cause it to wearexcessively. Therefore, at least 0. [5% sulfur and preferably about0.250.40 percent is included for best results. Above about 0.50 percent,not enough improvement is obtained to offset the accompanyingdisadvantages to warrant further additions of sulfur.

Chromium and molybdenum primarily contribute to the corrosion resistanceof our composition and, as ferrite formers, work to insure anessentially single phase ferritic microstructure in our composition inits annealed and quenched condition. To this end, both chromium andmolybdenum are limited to the critical ranges indicated. To provide thedesired resistance to corrosion in oxidizing media, at least about l7.5%chromium is required. Above about 19.5% chromium, insufficientimprovement in corrosion resistance is obtained, and larger additions ofchromium are not warranted. Beyond that, increasing chromium above 19.5percent objectionably affects the impact transition temperature bycausing it to rise, and also increases the tendency toward 885Fembrittlement. Best results are obtained with about l8.0 to 19.0%chromium.

Molybdenum enhances corrosion resistance and crevice corrosionresistance in reducing or pitting media, e.g., sulfuric acid, diluteferric chloride, or salt water. It also tends to raise the impacttransition temperature. For these reasons, molybdenum in an amountranging from about 1.5-2.5 percent is used and for best results aboutl.7-2.2% molybdenum should be used.

Our alloy is prepared and shaped using customary metallurgical practicessuitable for the making and shaping of ferritic stainless steelcontaining about 18% chromium. Hot working is carried out at about l,800to 2,200F, preferably between about l,900 to 2,lF, the higher hotworking temperature, e.g., from about l,950 to 2,200F being best suitedfor initially breaking down the composition containing the largeramounts of nitrogen.

Following hot working, the shapes are cooled in air, and some martensiteis formed, but at a temperature (M,,) below l,000F with the result thatthe intermetallic phases, e.g., nitrides and carbides are not formed,the carbon and nitrogen being retained in solution in the martensite(body centered tetragonal) formed from the austenite which had beenpresent at the hot work temperature. The material is heat treated byannealing at about l,200 to l,6()0F, preferably between about l,300 tol,500F, and better yet between about l,400 to l,500F. Depending upon thethickness of the section, the parts are held at the annealingtemperature for up to about 4 hours, shorter times of 1 to 2 hours orless being preferred. Following annealing and quenching, the martensiteis seen as highly tempered, not fully decomposed martensite. By highlytempered, not fully, that is partially, decomposed martensite is meantthe ferritic structure obtained by quenching from the annealingtemperature and in which no trace of the acicular structurecharacteristic of tempered martensite can be found, but the relativelyfine nitrides remain to identify the prior location of temperedmartensite. ln practice, an excessively high annealing temperature leadsto undesired agglomeration of the nitrides and other intermetallicphases that may be present.

Best results are obtained when the quenching following annealing iscarried out more rapidly than can be done in air except for relativelysmall cross-sections or for parts containing the larger amounts ofnitrogen with or without the larger amounts of carbon contemplatedherein. In any event, whether in air or in faster quenching media suchas oil or water, quenching should be sufficiently rapid to prevent orminimize precipitation of undesired phases, e.g., a plate-like nitride,in the grain boundaries.

Examples l-6 prepared as small experimental heats are illustrative ofour invention and had the composition indicated in Table l with thebalance iron and incidental impurities which included small amounts ofbut less than 0.005% phosphorus, less than 0.2% nickel, and less than0.2% copper.

TABLE I Ex. No. C Mn Si S Cr Mo N 1 .011 1.83 .40 .35 18.4! 2.01 .092 2.038 L .40 .35 18.44 2.01 .092 3 .057 1.110 .37 .34 18.43 2.02 .085 4.012 1.88 .41 .35 13.59 2.03 .139 5 .041 I88 .41 .35 [8.60 2.02 .139 s.056 [.83 .36 .35 18.45 2.01 .130

Examples l-6 were melted under an argon atmosphere to facilitatecontrolling the nitrogen content for experimental purposes. Examples lto 2 were poured and cast as ingots from the same heat, but with addedcarbon in Example 2. Similarly, Examples 3 and 6 and Examples 4 and 5were formed from split heats, the variation between Examples 3 and 6being in the nitrogen content and between Examples 4 and 5 being in thecarbon content.

The thus formed 2% inch ingots were forged from 2,000F to 1% inch squarebars, reheating being carried out at 178 inch square. After hot working,annealing for r hour at l,500F followed by water quenching was carriedout before the H4; inch square bars were shaped into test specimens.Standard tensile test specimens having a 0.252 inch gage diameter and al-inch gage length were formed as well as standard V-notch Charpy impacttest specimens. The results of room temperature (72F) mechanicalproperty tests are given in Ta bles ll and Ill. In Table ll, the 0.2percent yield strength in thousands of pounds per square inch is givenunder 0.2YS (KSl), the ultimate tensile strength is given under UTS(KS1), and the percent elongation and percent reduction in area aregiven under EL and RA respectively. The data tabulated is, in eachinstance, an

average of two tests.

TABLE II .2YS UTS Ex. No. (KSI) (KSI) if El '4 RA I 55.5 77 27 51 2 58.98L5 27.5 50.5 3 61 84.5 27.5 56 4 60.5 84 25.5 48 5 (1L5 27 53 6 02.5 ss20 49 TABLE III VNC IMPACT STRENGTH (Ff. LBS.)

HARDNESS 41) Ex. No.

The machinability of the specimens of each of the examples as annealedand quenched was determined as the average depth of penetration ininches into the specimens under carefully controlled conditions. Whilethere is no generally accepted standard for measuring machinability, thefree machining values were obtained by measuring the depth ofpenetration into the specimens by a quarter-inch drill in a timeinterval of seconds with the drill rotating at or very close to 670r.p.m. under constant torque. Before the start of each drillingoperation, the drill mounted in a conventional drill press was broughtagainst the surface of the specimen where it was maintained by aconstant weight of 100 pounds. The results of the tests are recorded inTable IV under drill penetration (DRILL PENE.) and each is an average oftwo sets of 3 hole tests. The carbon and nitrogen contents are includedfor easy reference.

To demonstrate the effect of nitrogen on the impact transitiontemperature, standard V-notch Charpy test specimens were prepared fromExamples 2 and 4 and tested at 32, 72, 150, and 200F. In addition,standard V-notch Charpy specimens were prepared for comparision of anAlloy C having the following composition:

w/o Carbon 0.037 Manganese l .92 Silicon 0.43 Sulfur 0.36 Chromium 18.00Molybdenum 2.05 Nitrogen 0.039

and the balance iron plus incidental impurities. These and the testspecimens yet to be referred to herein prepared from Alloy C were madefrom bars hot worked, annealed and quenched as was described inconnection with Examples l-6. Except in the case of Example 2 where thetransition temperature was determined by interpolation, thedetermination of the transition temperature was based on the measuredimpact strengths and the appearance of the fractures at the temperatureindicated, the impact transition tempera ture being based on the lowesttemperature to give a predominately ductile fracture appearance. Thetransition temperature for Example 2 was found to be at about ll0F, forExample 4 it was found to be about room temperature (72F), and for AlloyC it was found to be at about F.

It should also be noted that Alloy C tensile specimens when prepared andtested as was described in connection with Examples 1-6 gave a 0.2percent yield strength of 53 KSI, an ultimate tensile strength of 74.5KSl, an elongation of 29.5 percent and a reduction in area of 57percent. The annealed and quenched hardness was R,,90 and the V-notchCharpy impact strength was 7 ft. lbs., all being averages of two tests.

Corrosion rates of duplicate test coupons formed from the forged andheat treated bars prepared as was previously described herein of each ofExamples 1-6 and Alloy C and immersed for five 48-hour periods each inboiling 65% by weight nitric acid were found to have lost metal at thecalculated average rate in mils per year (MPY) as indicated in Table V.

From the data in Table V, it is apparent that the addition of nitrogenin accordance with the present invention does not adversely affect thecorrosion resistance of our composition in such media as 65 percent byweight boiling nitric acid.

Further important advantages of our composition reside in that itscorrosion resistance is at least as good as that of A.l.S.l. Type 303 asmeasured by such tests as salt spray corrosion resistance, resistance tonitric acid, and other media. The machinability of our composition issuperior to that of Type 303 both when measured by the drill test andalso when measured on a lathe.

When the unique combination of free machinability with good impacttoughness and corrosion resistance of our alloy is not required, theelements manganese and sulfur become optional and need not be present inamounts any greater than usual depending upon the manner in which thesteel is made. In the case of steel manufactured in air under a slag,manganese can be as low as 0.2 percent although even smaller amounts maybe present when vacuum melting practices are used. And sulfur would notexceed about 0.03 percent.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation. and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof,

but it is recognized that various modifications are possible within thescope of the invention claimed.

We claim:

I. A ferritic stainless steel consisting essentially in weight percentof about Carbon 0.08 Max. Manganese 2.5 Max. Silicon l Max. Phosphorus0.04 Max Sulfur 050 Max. Chromium l7.5-l9.5 Molybdenum 1.5-2.5 Nickel0.5 Max. Copper 0.5 Max. Nitrogen 0.06-0.20

the balance being essentially iron and incidental impurities, in whichselenium can be substituted for all or part of the sulfur on a l for lbasis and tungsten can be substituted for all or part of the molybdenumin the ratio of about 1.5% tungsten to l% molybdenum, and said stainlesssteel when annealed at about l,200-l,600F and quenched after having beenhot worked from about l,8002,2()0F contains about to 50% highly temperedpartially decomposed martensite free of acicular structure.

2. Ferritic stainless steel as set forth in claim 1 containing 0.25-0.4%sulfur.

3. Free-machining ferritic stainless steel as set forth in claim 2containing about 0.06% maximum carbon, about l.72.2% manganese and about0.08-0.l6% nitrogen.

4. Free-machining ferritic stainless steel as set forth in claim 3containing about LO-19.0% chromium.

5. Free-machining ferritic stainless steel as set forth in claim 3containing about l.72.2% molybdenum.

6. Free-machining ferritic stainless steel as set forth in claim 4containing about l.7-2.2% molybdenum.

7. Free-machining ferritic stainless steel as set forth in claim 6containing about ODS-0.14% nitrogen.

8. Free-machining ferritic stainless steel as set forth in claim 7containing about 5 to 20 percent highly tempered partially decomposedmartensite.

9. Free-machining ferritic stainless steel as set forth in claim 3containing about 1.8% manganese, about 0.4% silicon, 0.35% sulfur, aboutl8.4 to l8.6% chromium, about 2% molybdenum, about 0.085-0.l4% nitrogen,and about 5 to 20 percent highly tempered partially decomposedmartensite.

l0. Free-machining ferritic stainless steel as set forth in claim 9containing about 0.l300.l40% nitrogen.

II. A stainless steel article formed from a ferritic stainless steelhaving the composition set forth in claim 1 hot worked from aboutl,8002,200F, annealed at about l,200l ,600F and quenched so as tocontain about 5 percent to 50 percent highly tempered, partiallydecomposed martensite free of acicular structure.

12. The stainless steel article set forth in claim I] formed from aferritic stainless steel containing O.250.4% sulfur, about l.7-2 .2%manganese, about 0.08-0.l6% nitrogen, about l8.0-l9.0% chromium, about1.7-2.2% molybdenum, a maximum of about 0.06% carbon, andabout 5 to 20percent highly tempered, partially decomposed martensite free ofacicular structure.

1. A FERRITIC STAINLESS STEEL CONSISTING ESSENTIALLY IN WEIGHT PERCENTOF ABOUT
 2. Ferritic stainless steel as set forth in claim 1 containing0.25-0.4% sulfur.
 3. Free-machining ferritic stainless steel as setforth in claim 2 containing about 0.06% maximum carbon, about 1.7-2.2%manganese and about 0.08-0.16% nitrogen.
 4. Free-machining ferriticstainless steel as set forth in claim 3 containing about 18.0-19.0%chromium.
 5. Free-machining ferritic stainless steel as set forth inclaim 3 containing about 1.7-2.2% molybdenum.
 6. Free-machining ferriticstainless steel as set forth in claim 4 containing about 1.7-2.2%molybdenum.
 7. Free-machining ferritic stainless steel as set forth inclaim 6 containing about 0.08-0.14% nitrogen.
 8. Free-machining ferriticstainless steel as set forth in claim 7 containing about 5 to 20 percenthighly tempered partially decomposed martensite.
 9. Free-machiningferritic stainless steel as set forth in claim 3 containing about 1.8%manganese, about 0.4% silicon, 0.35% sulfur, about 18.4 to 18.6%chromium, about 2% molybdenum, about 0.085-0.14% nitrogen, and about 5to 20 percent highly tempered partially decomposed martensite. 10.Free-machining ferritic stainless steel as set forth in claim 9containing about 0.130-0.140% nitrogen.
 11. A stainless steel articleformed from a ferritic stainless steel having the composition set forthin claim 1 hot worked from about 1,800*-2,200*F, annealed at about1,200*-1,600*F and quenched so as to contain about 5 percent to 50percent highly tempered, partially decomposed martensite free ofacicular structure.
 12. The stainless steel article set forth in claim11 foRmed from a ferritic stainless steel containing 0.25-0.4% sulfur,about 1.7-2.2% manganese, about 0.08-0.16% nitrogen, about 18.0-19.0%chromium, about 1.7-2.2% molybdenum, a maximum of about 0.06% carbon,and about 5 to 20 percent highly tempered, partially decomposedmartensite free of acicular structure.